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Evolution of the natural pasturelands of C.N.R.Z., Sotuba *

(*) Report of the Centre National de Recherche Zootechnique de Sotuba.


I. Climatology
II. Geomorphology and geology
III. Pedology
IV. Plant formations
V. Productivity of certain rangelands
Conclusions


SUMMARY

Based on the study of Sotuba's rangelands carried out in 1959 and 1974, this paper attempts to give an account of the evolution of the vegetation over the last 15 years. It then indicates the primary production of these rangelands where there has been no definite management policy and attempts to define a rational method of exploitation.

Studies of the natural pasturelands of C.N.R.Z. at Sotuba, which is considered to be typical of Mali's Sudanian zone, were carried out in 1959 and 1974.

The work in 1959 resulted in a detailed pedological study by C. Charreau and Y. Dommergues. This study was followed by a floristic inventory carried out by J.G. Adam, and experimental studies on the exploitation and improvement of these pasturelands by Z. Derbal, J. Pagot and J. Lahore.

In 1974, the studies were all concerned with the principal plant formations, the aim being to determine if, since the initial studies, the vegetation had undergone a process of evolution.

I. Climatology

The station's climate is of the Sahelo-Sudanian type. The following table makes it possible to compare the data of 1959 with that of 1974.

Years

Temperatures (°C)

Rainfall

Max. average

Min. average

in mm

1959

34.4

19.8

1,164.95

1974

37.4

16.2

958.08

II. Geomorphology and geology

Sotuba is situated in a zone having two sandstone series, which are:

a) the Koulouba schistous sandstone series bounded at the north by the alluvial plain, which it overhangs at a height of 150 m;

b) the so-called Sotuba sandstone series, buried beneath the alluvial deposits of the plain. This formation becomes visible on the river bed at the level of the chaussée Coursin.

The first series, which dates from the Ordovician age, includes several sandstone layers, Harlania alternating with two layers of intercalary shale. These sandstones are covered by a strong protective ferruginous layer. The Sotuba series, situated below the first, dates from the Cambrian age.

III. Pedology

The 1959 studies highlighted 4 types of soil:

1) glacis de piémont soils, which are skeletal soils;

2) emerged soils of the alluvial plain, which are advanced, weathered, tropical ferruginous soils;

3) the soils of the principal layer, which are either hydromorphic soils temporarily blocked from the surface, or hydromorphic soils permanently blocked from the surface;

4) soils of altered formations bordering the river.

IV. Plant formations

The major corresponding plant formations are, respectively:

1) An open formation of shrubby savannah with a small number of woody species, among which Pterocarpus lucens is noted (type 1). The herbaceous stratum included two "Arundinelus" graminaceae: Loudetia togoensis (type 3/3), and Danthoniopsis sp. (type 1/4), and a "Chloridus" Schoenefeldia gracilis (type 2/3).

2) A dense savanicole formation with a great variety of woody species: this is a formation containing Vitellaria paradoxa (type 4/3). In the herbaceous stratum, Andropogon gayanus and Cymbopogon giganteus (both type 4/3) are dominant.

3) A grassland of Andropogon grasses and Cyperaceae with the species Hyparrhenia sp. (1/2), Panicum anabaptistum (2/2) and Eragrostis tremula (3/2).

4) Humid grasslands of Oryza barthii occurring in the flood zone of the river.

5) A wooded savannah situated on the folds of the river bank, characterised by thorny bushes and the companion species Ziziphus and Ficus.

6) A wet grassland situated on the bed of the river, with a predominance of Vetiver.

The 1974 studies highlighted 11 plant groups, as follows:

2 on cuirass
4 on advanced tropical ferruginous soil
1 on re-formed sandy alluvial soil
2 on collu-alluvial soil
2 in the hydromorphic zone

The formations on the cuirass soil show a predominance of Pterocarpus lucens and Lamnea acida in the wooded stratum, Combretum ghasalense in the shrubby stratum, and Andropogon pseudapricus in the herbaceous stratum (type 4).

On the advanced tropical ferruginous soils Pteleopsis suberosa, Terminalia avicinoides and Andropogon gayanus are dominant. However, it should be pointed out that in enclosures 1 and 2 of type B1, planning work encouraged a great expansion of Guiera senegalensis. The predominance of Andropogon gayanus in type B1 enclosures would seem to indicate some evolution of the vegetation towards the establishment of perennial species. In respect of woody species, the presence and abundance of Guiera senegalensis indicates that the woody cover that was suppressed at the time of the setting up of the enclosures had to a certain extent returned.

For the B2 formation of enclosure 12, the presence of Sporobolus pyramidalis seems to be linked to the degraded condition of the pasturelands of this zone. In fact, this enclosure had been protected from grazing in 1973 after overgrazing had occurred, causing its degeneration.

Another indication of the evolution of the flora of the savanicole formation is the appearance of flora species of the humid zones on the cultivated plots. The causes of this appearance must be sought in the inefficient drainage of the plots. The species encountered include:

Setaria pallidifusca
Paspalum scrobiculatum
Panicum subalbidum
Eleusine verticillata
Cyperus swein furthianus, and so on.

V. Productivity of certain rangelands

Estimates of primary productivity, carried out by students of the C.P.S. (*) on certain of Sotuba's enclosures, produced the following results:

(*) Centre Pédagogique Supérieur.

Plots

Productivity in T/ha of DM

1973

1974

A1

1.66

2.9 ±0.81

B1 (1)


3.4 ± 0.81

B1 (4)


3.5 ± 1.69

B1 (6)

3.64


B2 (8)

1.54

3.6 ± 0.78

B12 (12)


3.0 ± 1.00

B3 (14)


2.1 ± 0.43

H1

4.42

6.3 ± 1.04

T2


5.1 ± 0.82

This table is not complete and estimates should take into consideration several years in order to make it possible to draw conclusions concerning the evolution of the productivity of the pasturelands. It does, however, show that because of a higher rainfall in 1974 (1,127.5 mm as against 980.6 mm in 1973), the pastureland had a higher level of productivity.

COMPARATIVE TABLE OF FORMATIONS

Substratum

Formation

Species

Type

Observations

1959

1974

Glacis d'érosion: Skeletal soils, collualluvial/cuirass, and broken-up cuirass Groups A2, B3

Shrubby, open savannah with a small number of woody species

P. lucens

1/1

1

Non-flourishing appearance of P. lucens, Danthoniopsis; and Schoenefeldia disappeared for the moment. On broken-up cuirass (P7) A. gayanus seems to have replaced A. pseudapricus.

Loudetia togoensis

3/3

3

Danthoniopsis

1/4

-

Schoen. gracil.

2/3

-

A. pseudapr.

5/4

4

Alluvial plain: Emerged zone B1, B2

Dense Savannah. with a large number of woody species

V. paradoxa

4/3

1

The karite seem to have disappeared or to be disappearing. There is recolonization by woody species following management activities.

A. gayanus

4/3

4

Guiera seneg.

3/3

3

Pteleopsis

2/2

2

Indigofera pan.

1/1

2

Hydromorphic zone Alluvions and collualluvions (fine) Group H1

Grasslands of Andropogons and Cyperaceans

S. trichopus


4

The hydromorphic zone seems to have undergone considerable colonization by Sorghum trichopus

Conclusions

The following conclusions can be drawn from this comparative study:

1) Since the time of the initial studies which revealed that Andropogon pseudapricus formed the basic cover of the flora of the pasturelands of C.N.R.Z., this species has clearly undergone a regression in certain sectors (for example: enclosures of the type B1, where Andropogon gayanus has taken the place of Andropogon pseudapricus).

2) A phenomenon of overrunning by bushy species in the plant groups on the emerged soil can be noted. These species include in particular:

Guiera senegalensis
Pteleopsis suberosa
Daniellia oliveri
Combretum lecardii, and so on.

The phenomenon of the expansion of bushy species appears to have been encouraged by brush fires, which have occurred periodically in the enclosures.

3) The planned exploitation of the pasturelands by means of the system of bands alternately cleared of bush and not cleared of bush also encourages the overrunning by bushy species, that is, unless the bush has been completely cleared and normal exploitation resumed. Such exploitation might take the form of alternating cutting and grazing; this process would make possible the best use of the herbaceous resources of the pasturelands.


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